1. Field of Invention
The present invention relates to an electronic ballast. More particularly, the present invention relates to an electronic ballast capable of generating an asymmetric driving signal.
2. Description of Related Art
Recently, a gas-discharge fluorescent lamp has become widely used illumination equipment. The fluorescent lamp have better luminous efficiency than a conventional incandescent bulb, because a higher proportion of energy consumed by the fluorescent lamps is converted into visible light and also less heat is generated by the fluorescent lamp. Therefore, the operating temperature of the fluorescent lamp is lower than the operating temperature of the conventional incandescent bulb under the same brightness. The fluorescent lamp with better luminous efficiency is utilized in a variety of illumination applications.
The conventional fluorescent lamp needs to work with an electronic ballast. Reference is made to FIG. 1, FIG. 1 is a schematic diagram illustrating a conventional electronic ballast 100. The electronic ballast 100 includes a direct current power source 180, a rectifier circuit 190, an inverter 160, a transformer 120 and a resonance circuit 140.
However, a striation phenomenon is likely to occur in the fluorescent lamp under low temperature (for example, when the fluorescent lamp is just activated) or low power (for example, when the fluorescent lamp is set to be operated under low brightness). Reference is made to FIG. 2, which is a schematic diagram illustrating a conventional fluorescent lamp 200 with the striation phenomenon. The conventional fluorescent lamp 200 with the to striation phenomenon will flicker between darkness and brightness, thus affecting user visual experience. Furthermore, the striation phenomenon on the fluorescent lamp 200 under low power also limits the adjustable dimming range of the fluorescent lamp 200.
The conventional solutions includes adding a direct current (DC) component into the driving signal of the inverter 160 of the electronic ballast 100, or providing an asymmetric waveform as the driving signal, thereby solving the striation phenomena on the fluorescent lamp 200.
Referring to FIG. 3 for adding a direct current, FIG. 3 is a schematic diagram illustrating a DC component 104 added into a driving signal waveform 102. However, in the driving signal waveform 102 as shown in FIG. 3, the DC component 104 will continuously consumes one single side of the filament material within the fluorescent lamp, such that the lifetime of the fluorescent lamp will be reduced.
In another solution, an asymmetric waveform is provided as the driving signal. Referring to FIG. 4, FIG. 4 is a schematic diagram illustrating a conventional asymmetric waveform 106. The asymmetric waveform 106 can be generated by an asymmetric driving circuit. However, the asymmetric driving circuit may drive one of power switch units (such as a bipolar junction transistor) within the electronic ballast of the fluorescent lamp into an over saturation state. The power switch unit in the over saturation state may increase the switching loss and elevate the operating temperature.